Organic Composite Modification And Gas Sensing Properties Of One-Dimensional Silicon-Based Arrays

Silicon nanowires are a kind of typical one-dimensional silicon-based structural material.Silicon nanowires formed by metal-assisted chemical etching（MACE）technology have high specific surface area and are highly directional and highly compatible with the silicon-based semiconductor process.In particular,the sensing characteristic reflecting at room temperature makes it promising in the field of low-power integrated gas sensors.To enhance the sensing response of silicon nanowires when operating at room temperature,surface modification has proven to be an effective way.In this work,the modification of silicon nanowires by organic polymer is performed to improve their gas response performance.The organic-inorganic heterogeneous effect is realized by controlling the modified/coated composite structure of the organic conductive polypyrrole on the surface of ordered silicon nanowires.As a result,the gas sensing performance of the silicon nanowire-based gas sensor is significantly improved.The gas-sensing mechanism of the organic-inorganic composite structure is clarified by establishing a corresponding theoretical model.Firstly,a two-step MACE（dual-MACE）process,which can realize the effective microstructure modification of silicon nanowire arrays,is developed.In this process,the MACE-produced Ag is used to realize the secondary MACE for the as-formed silicon nanowire array.After dual-MACE,a silicon nanowire array with loose structure and rough surface is achieved.The dual-MACE process significantly increases the adsorption surface of gas molecules on silicon nanowires,and simultaneously creates a high-density unstable surface state on the nanowires,thus promoting the surface adsorption of gas molecules obviously.Meanwhile,the dual-MACE process decrease the density of nanowire arrays,facilitating further surface modification and internal diffusion of gases.Secondly,based on the loose,rough silicon nanowire array from dual-MACE,a polypyrrole film/silicon nanowire core-shell array structure with ultra-thin shell（10nm）was successfully prepared by gas phase chemical polymerization,while a polypyrrole nanoparticle-modified silicon nanowire array was achieved by embedding a large amount of 20-50 nm highly dispersed polypyrrole nanoparticles in the rough surface of the dual-MACE-produced silicon nanowires.The sensing performances of the two composites to NH₃ were studied comparatively at room temperature.It is found that the response values are increased by about 27 times and 6 times for the core-shell structure and the modified one to 10ppm amonia gas,respectively,compared with the bare silicon nanowire arrays.The sensor based on nanowire/polypyrrole film core-shell structure exhibits faster dynamic characteristics and lower detection limits than the polypyrrole particles-modified nanowires.At room temperature,the response time of the core-shell sensor to 1-10 ppm NH₃ is less than 3s,and the NH₃ detection limit can reach 180 ppb.Finally,the polypyrrole/silicon nanowire core-shell structures with different polypyrrole shell thickness were prepared by controlling the gas phase polymerization time.Microstructure characterization and NO₂ gas sensitivity evaluation were carried out.By establishing a theoretical model,a mathematic formula,which can explain experimental results reasonably,was analyzed and derived.Specially,the resulted theoretical model is universal and can provide valuable guidance for studies on gas sensing systems based on other one-dimensional organic/inorganic composites.